Image recording material containing titanium dioxide,and zinc oxide in minor amount and the use thereof

ABSTRACT

AN IMPROVED TITANIUM DIOXIDE PHOTORECEPTOR INCLUDES A MINOR AMOUNT OF ZINC OXIDE. IT HAS BEEN FOUND THAT PREFERABLY AN AMOUNT OF UP TO ABOUT 10% ZINC OXIDE IN A TITANIUM DIOXIDE OR OTHER PHOTOCONDUCTOR LAYER MORE THAN DOUBLES THE PHOTOSENSITIVITY. THE IMPROVED LAYER IS WELL SUITED TO A KNOWN PHOTOLYTICALLY INITIATED PROCESS.

United States Patent O 3 706,560 IMAGE RECORDING MATERIAL CONTAINING TITANIUM DIOXIDE, AND ZINC OXIDE IN MINOR AMOUNT AND THE USE THEREOF Clark F. Grain, Framingham, Mass., assignor to Itek Corporation, Lexington, Mass. No Drawing. Filed July 21, 1970, Ser. No. 56,944 Int. Cl. G03c 5/24, 1/00 US. CI. 96-48 21 Claims ABSTRACT OF THE DISCLOSURE An improved titanium dioxide photoreceptor includes a minor amount of zinc oxide. It has been found that preferably an amount of up to about zinc oxide in a titanium dioxide or other photoconductor layer more than doubles the photosensitivity. The improved layer is well suited to a known photolytically initiated process.

BACKGROUND OF THE INVENTION Data or image storage media comprising radiationsensitive materials such as titanium dioxide are described in detail in US. Pats. 3,152,903; 3,052,541; French Pats. 345,206 and 1,245,215 and in British Pat. 1,043,250. In the aforementioned British patent, radiation-sensitive titanium dioxide functions as a photoconductive component of the media and exposure of said media to activating means such as radiant energy, electron beams or the like results in the storage of a reversible latent image pattern therein. The reversible latent image pattern exists for a limited time during which said pattern can be converted to an irreversible form and read out visually by contacting said pattern with a suitable image forming material, such as a chemical redox system. In the aforesaid U.S. and French patents, the radiation-sensitive material is combined with at least one component of an image-forming material prior to exposure to activating means. For example, US. Pat. 3,052,541 describes a photoconductive copy media comprising a photoconductive material such as titanium dioxide in combination with a. reducible metal ion such as silver nitrate. This copy media is exposed to activating means and then contacted with a reducing agent to produce a visible image. On the other hand, US. Pat. 3,152,903 discloses a system wherein the photoconductive material is used in combination with both an oxidizing agent such as silver nitrate and a reducing agent such as hydroquinone. Upon exposure to suitable activating means, a visible image is formed. One of the limitations of the above-mentioned data or image storage systems is that they lack the photographic speed of systems such as silver halide. Therefore, in order to expand the possible uses of these photographic systems described in the above-mentioned patents and application, it is highly desirable to increase the photographic speed of the systems. Much research effort has been spent in trying to find ways to increase speed.

In US. 3,471,288 there is disclosed one embodiment of this new photographic system which employs a layer containing titanium dioxide or a layer containing zinc oxide and which may employ a mixed layer containing a major proportion, for example 85%, of zinc oxide and a minor proportion, for example of titanium dioxide.

GENERAL STATEMENT OF THE INVENTION It has now been found that a photoconductive layer of improved sensitivity can be produced by the combined use of a major amount of titanium dioxide, 50% or greater, and preferably greater than 90%, based on the total weight of active photoconductor, together with a minor amount of zinc oxide, and preferably less than 10%,

based on the total weight of active photoconductor. The two materials are mixed together in a binder and coated on a support base such as paper, film or the like to produce an improved photosensitive product.

The new product containing the mixture of a major proportion of titanium dioxide and a minor proportion of zinc oxide can then be employed according to British Pat. No. 1,043,250 and other known photorecording methods and systems. According to one form of such process, the photosensitive product is exposed to a photographic image which is to be recorded, and, if desired, to produce a permanent image, treated with a reducible metal ion such as, for example, copper, silver, gold or the like. The treatment may be accomplished for example by applying to the surface a solution of the appropriate metallic ion as for example, a solution of silver nitrate. There is, thereupon, formed a chemical latent image which is capable of being developed by means of a photographic reducing developer.

Generally, an amount of less than 10% zinc oxide based on the total weight of zinc oxide and titanium dioxide is employed, since excessive fogging occurs unless special developers are used or other special precautions are taken. It has been found for example, that a layer containing 2% zinc oxide and 98% titanium dioxide is about three times as sensitive to incident light as is a layer consisting of titanium dioxide in the absence of zinc oxide.

One of the advantages of this invention is in certain photographic processes using a copy medium comprising titanium dioxide and a sensitizing dye which serves (1) to sensitize the titanium dioxide to different wavelengths of light, such as visible light, and, (2) to increase the photographic speed of the copy medium. By means of the present invention the photographic speed can be maintained at the same level, or a predetermined level, while significantly reducing the amount of sensitizing dye being used per unit area of copy medium. This has the advantage of providing a more economical copy medium and also lessens the amount of dye material which sometimes adds to the sludging problem in the processing.

DETAILED DESCRIPTION OF THE INVENTION In accordance with this invention, a number of coating formulations have been prepared and these formulations have been tested to determine the increase in photosensitivity in relation to the composition of the coating and particularly in relation to the amount of zinc oxide compared with the amount of titanium dioxide used in test materials.

It has been determined that a mixture containing 0.5% zinc oxide and 99.5% titanium dioxide has a substantial increase in photographic speed compared with a similar material having 100% titanium dioxide as its photoconductive agent. The difference in photosensitivity is somewhat greater than two fold with the use of /2% zinc oxide. With 1% and 2%, respectively, of zinc oxide, the photosensitivity is increased by at least three times without significant increase in background fog or other detrimental properties. Amounts of zinc oxide up to and in excess of 5%, based on the total weight of pigment, have been employed, also with gratifying results. When 5% zinc oxide is employed, there continues to be at least a three fold increase in photographic sensitivity accompanied by a moderate but acceptable increase in fog. When 10% Zinc oxide is used together with titanium dioxide, the fog or background rises perceptively and for general application purposes, reaches an undesirable level.

The titanium dioxide preferred is a photosensitive material having an average particle size less than about 250 millimicrons. Preferably the titanium dioxide is produced by high temperature pyrolysis of titanium halide.

The copy medium of this invention may be sensitized to visible or other wavelengths of light by means of sensitizing dyes such as the cyanine and hemicyanine dyes, such as described in commonly owned co-pending U.S. Ser. No. 633,689, filed Apr. 26, 1967, and U.S. Ser. No. 796,167, filed Feb. 3, 1969.

The photosensitive material of this invention may be used on a number of different substrates such as paper, film, or metal supports. The photosensitive material may be incorporated in this support or may form a separate layer on the surface of the support. Especially desired film bases are cellulose acetate or polyester, such as polyethylene terephthalate and especially a film base having a subbing layer to improve the adhesion of the photosensitive layer to the film support. Also an especially desired substrate when this invention is used for printing plates or name plates is a roughened metallic support such as a grain, anodized aluminum support.

The thickness of the photosensitive layer or the imageforming layed will depend upon the use to which the copy medium is to be used, the nature of the binder, where present, the amount of activating radiation utilized, and other like factors. In order to obtain rapid processing and images which are adherent to the support, it is often desirable to have relatively thin photosensitive layers preferably less than two microns and more preferably less than about one micron in thickness. However, the thickness of the photosensitive layer may be varied according to the effects desired.

When used, the amount of binder to amount of photoconductor or other photosensitive material may vary over wide ranges. Preferably, from about one part by weight to about six parts by weight, and more preferably four parts by weight of photosensitive material per part by weight of binder will be used.

The titanium dioxide or zinc oxide may be sensitized to visible and other wavelengths of light by foreign ion doping or by addition of fluorescent materials.

Irradiation sources which are useful in this invention for producing the initial latent image include any of the usual irradiation means commonly used with the selected Photosensitive material, Thus, actinic light, X-rays, or gamma rays are effective. Beams of electrons or other light particles may also be used in the place of ordinary forms of electromagnetic radiation for forming an image. These various activating means are designated by the term activating radiation.

Binders useful for the present invention can comprise any of the wide variety of materials known in the photographic art. In general, these binders are transluscent or transparent so as not to interfere with transmission of light therethrough. They are desirably also solvent permeable in order to allow rapid physical development to take place. Preferred binder materials are organic materials such as natural or synthetic polymers. Examples of suitable synthetic polymers are butadiene-styrene copolymer, poly(alkyl acrylates) such as poly( methyl methacrylate), polyamides such as polyacrylamide, polyvinyl acetate, polyvinyl alcohol and polyvinylpyrrolidone. Natural polymers such as gelatin are also useful. When it is desired to remove the binders it may be desirable to have them solvent soluble in order that they may be readily washed ofl after development of the image has taken place. Preferably in such solvent soluble systems the binder should be removable with aqueous systems.

The photosensitive material of this invention should be conditioned for exposure as by storing in the dark for from one to twenty-four hours prior to use, heating or other conditioning known to the art.

The period of exposure to form the latent image 'Will depend upon the intensity of the light source, particular photosensitive material, the type and amount of catalyst, if any, and light factors known to the art. In general, however, the exposure may vary from about 0.001 second to several minutes.

Suitable image forming materials are intended to include physical developers such as described in U .8. Pat. 3,152,903, and British patent specification No. 1,043,- 250, and British Pat. 1,064,725. These image-forming materials include preferably an oxidizing agent and a reducing agent. Such image-forming materials are also often referred to in the art as electroless plating baths. Electrolytic development such as taught in US. Pat. 3,152,969, can also be used. The oxidizing agent is generally the image-forming component of the image-forming material. Either organic or inorganic oxidizing agents may be em ployed as the oxidizing component of the image-forming material. The oxidizing and reducing agent may be combined in a single processing bath, may also be in separate baths or one or both of these components may be incorporated in the imaging medium prior to exposure. Preferred oxidizing agents comprise the reducible metal ions having at least the oxidizing power of cupric ion and include such metal ions as Ag+, Hg, Pb+ Au+ Au+ Pt, Pt, Ni, Sn+ Pb, Cu+ and Cu.

The reducing agent component of the image-forming material are inorganic compounds such as oxalates, formates and ethylenediaminetetraacetate complexes of metal having variable valences; and organic compounds such as dihydroxybenzenes, aminophenols, and aminoanilines. Also, polyvinylpyrrolidone, hydrazine, and ascorbic acid may be used as reducing agents in this invention. Suitable specific reducing compounds include hydroquinone or derivatives thereof, oand p-aminophenol, pmethylaminophenol sulfate, p-hydroxyphenyl glycine, oand p-phenylenediamine, 1-phenyl-3-pyrazolidone, alkali and alkaline earth metal oxalates and formates.

The invention above described is exemplified as follows:

EXAMPLE 1 A dispersant solution is prepared consisting of 7.8parts Tamol 850 and 2.4 parts of potassium tripolyphosphate, both of which are known dispersant materials. These dispersants are dissolved in 1,000 parts of water.

A mix was prepared containing 1200 parts, by weight, of photoconductor material and 6,000 parts, by weight, of the aqueous dispersant solution. It was thoroughly mixed for five minutes, after which these was added to the mix 1200 parts, by weight, of a 25% solution of polyvinyl alcohol. After thorough mixing, the pigment binder mix was coated on a baryta base stock and dried.

The coated paper was tested for photosensitivity by exposure on a Kodak 101 sensitometer. It was then developed or processed according to the method disclosed in British Pat. No. 1,043,250. This process included immersion in a solution of silver nitrate followed by image development with a metol developer to form a permanently visible image. For purposes of comparison, the methods and processes were kept uniform except for the percentage of zinc oxide contained in the photoconductor material.

The tests of four properties in the final image are recorded herein. The first, identified as LES 10 is a measure of the reciprocal of exposure time required to produce 0.2 density units above fog and base. The second property measure, PS (or print speed) is the reciprocal of the exposure required to produce 0.6 density. The third property is Dmax. and the fourth property is the sum of base and fog in background areas. The results are set forth in Table I.

In Table I, it is shown that basic sensitivity (LESX10 is doubled by the inclusion of 0.5% zinc oxide and is multiplied to about six times by the inclusion of 2.0% zinc oxide, while at the same time, PS was increased by a factor of at least three with 0.5% zinc oxide and almost ten with 2.0% zinc oxide. Simultaneously, Dmax. was improved slightly and the base and fog remained essentially at the same level as a control sample, except that there was a moderate increase in base and fog with 2.0% zinc oxide.

The procedure of Example 1 was repeated with 5.0% zinc oxide and 95% titanium dioxide. The sensitivity of the coated paper was essentially the same as the sensitivity with 2.0% zinc oxide, although the base and fog in background areas was significantly higher. In a similar test with 10% zinc oxide, fog was too high.

EXAMPLE 3 The procedure of Example 1 was repeated, employing 2% zinc oxide and 98% titanium dioxide in a coating mix. The coating was placed on a transparent film base. The photosenstivity was sharply increased by a factor greater than five with reference to a control coating containing no zinc oxide. Dmax. was increased, and base and fog was only moderately higher.

EXAMPLE 4 A mixture of four parts by weight of Ti and one part by weight of an emulsion containing about 25% of solids is used to coat paper sheets. The paper is sensitized to visible light by incorporation into the emulsion 0.4% by weight of a sensitizing dye, 2-p-dimethylaminostyryl- S-methyl-1,3,4-thiadiazole-fl-hydroxy-etho chloride based upon the weight of titanium dioxide in the emulsion.

The dry sensitized paper is exposed for about 20 seconds to an image pattern of activating radiation by means of a 500 watt tungsten filament light source of an Itek RS Reader Printer to provide an exposure of 90 to 140 foot/candles of light.

The exposed paper is then developed by chemically processing at a speed of 4 /2 inches per second. The paper is first contacted with a 0.3 M aqueous solution of silver nitrate by means of a roller having a diameter of one inch and which contacts the soltuion and the exposed surface of the paper. The paper then moves to a developer station which consists of two one inch rollers spaced about 1% inches apart. These rollers contact the exposed surface of the paper and an aqueous developer solution comprising:

Grams Metol 20 Sodium sulfite 20 N,N-paraphenylenediamine sulfate 1 Ethylene thiourea 0.1 Nonionic surfactant 1.05 Sodium phosphate dibasic 18.7

Water to 1 liter.

The paper is then contacted with a stabilizer solution by means of a dip station about four inches in length. The stabilizer bath comprises an aqueous solution of the following composition:

Grams Potassium sulfite (K 80 200 Sodium thiosulfate (Na S O -5H O) 200 Acetamide 100 processing conditions are otherwise identical, a final print is produced having a print speed of 3.8 and a Dmax. of 1.2 is produced. Therefore, it can be seen that by incorporating zinc oxide in the titanium dioxide containing copy medium, the sensitivity is sufficiently increased that it is possible to decrease the amount of sensitizing dye by 50% to obtain the same Dmax.

The coated film and coated paper prepared in accordance with the examples can be processed by laboratory techniques to produce satisfactory images or pictures. The products can also be processed in automated equipment such as in Example 4 to produce superior results.

In the preparation of a coating mix for coating the binder pigment material on a support base, it is generally desirable to employ a material to aid dispersion, such as a conventional dispersant. Generally, dispersants which are photographically inactive and chemically neutral with respect to the other ingredients in the mix are employed.

What is claimed is:

1. An imaging medium comprising a photoconductive zinc oxide and a photoconductive titanium dioxide and wherein the zinc oxide is present in a minor amount and the titanium dioxide is present in a major amount, of 50% or greater, by weight based upon the total weight of zinc oxide and titanium dioxide.

2. An improved imaging medium comprising a support and a photoconductor thereon, said photoconductor comprising between about 0.5 and 10% zinc oxide based on the total weight of titanium dioxide and zinc oxide and the remaining to 99.5% titanium dioxide.

3. The imaging medium of claim 2. wherein the photoconductor comprises 0.5 to 5% zinc oxide and the remainder comprises titanium dioxide.

4. The imaging medium of class 3 wherein the photoconductor comprises 0.5 to 2% zinc oxide and 99.5% to 98% titanium dioxide.

5. An imaging medium as in claim 1 wherein the titanium dioxide has an average particle size of less than about 250 millimicrons.

6. An imaging medium as in claim 4 wherein the titanium dioxide has an average particle size of less than about 250 millimicrons.

7. An imaging medium as in claim 1 additionally comprising a sensitizing dye.

8. An imaging medium as in claim 2 additionally comprising a sensitizing dye.

9. An imaging medium as in claim 2 additionally comprising imageaforming materials comprising a reducible metal ion, a reducing agent for said metal ion or both of these components.

10. An imaging medium as in claim 2 wherein the support base comprises paper, film or metal.

11. An imaging medium comprising a support having thereon a photosensitive emulsion comprising a photoconductor, a sensitizing dye and a binder, said photoconductor comprising between about 0.5 and 10% zinc oxide based on the total weight of the zinc oxide and titanium dioxide and the remaining 90% to 99.5% being titanium dioxide.

12. -An imaging medium as in claim 11 wherein the titanium dioxide has an average particle size of less than about 250 millimicrons and wherein the photosensitive emulsion additionally comprises a surfactant.

13. An imaging medium comprising a support having thereon a photosensitive emulsion comprising 0.5 to 2% zinc oxide and 99.5 to 98% titanium dioxide based on the total weight zinc oxide and titanium dioxide, a cyanine or hemicyanine sensitizing dye, and a binder, said photosensitive emulsion forming a layer having a thickness less than about two microns.

14. In the photographic process comprising imagewise exposing to activating radiation an imaging medium comprising a photosensitive emulsion comprising zinc oxide and titanium dioxide, the improvement wherein the zinc oxide is present in a minor amount and the titanium di- 7 oxide is present in a major amount, of greater than 50%, by weight based on the total weight of zinc oxide and titanium dioxide.

15. A process as in claim 14 comprising additionally contacting the imaging medium with image-forming material which undergoes an oxidation-reduction type reaction upon contact with an exposed, activated photoconductor to produce a visible image.

16. A process as in claim 15 wherein the image-forming material comprises a solution of reducible metal ions.

17. In a photographic process comprising exposing to a pattern of visible light an imaging medium comprising a support base and a photosensitive emulsion comprising photoconductive zinc oxide and titanium dioxide, the improvement wherein the zinc oxide is present in an amount between about 0.5 and 10% by weight and the titanium dioxide being present in an amount between about 90.0 and 99.5% by weight based upon the total weight of zinc oxide and titanium dioxide, and a sensitizing dye.

18. A process as in claim 17 additionally comprising 20 exposure and wherein the image-forming material comprises a solution of reducible metal ions and a reducing agent for said metal ions.

20. A process as in claim 19 wherein the solution of reducible metal ions comprises silver ions in a separate bath and wherein the reducing agent comprises metol in a separate bath.

21. A process as in claim 20 additionally comprising contacting the developed imaging medium with a photographic fixing or stabilizing bath.

References Cited UNITED STATES PATENTS 3,561,968 2/1971 Dantro 96-88 3,471,288 10/1969 Berman 96-48 PD 3,052,541 9/1962 Levinos 96-48 PD FOREIGN PATENTS 1,482,724 6/1966 France 96-48 PD NORMAN G. TORCHIN, Primary Examiner W. H. LOUIE, 1a., Assistant Examiner US. Cl. X.R.

Disclaimer 3,706,560.-0la1'7c F. 6 min, Framingham, Mass. IMAGE RECORDING MA- TERIAL CONTAINING TITANIUM DIOXIDE AND ZINC OXIDE IN MINOR AMOUNT AND THE USE THEREOF. Pattent dated Dec. 19, 197 2. Disclaimer filed Feb. 6, 197 4:, by the assignee,

I tek Gwpomtion.

Hereby enters this disclaimer to claims 1, 7, an

[Ofilcial Gazette Mawch 11, 1975.]

d 14 of said patent. 

